Determining the measurment noise matrix of the Pixhawk for the IMU

edit

we are currently working on developing an AUV in which we utilize the pixhawk for its IMU then using those sensor reading in a SLAM Algorithm.

but an integral part of the EKF Slam or any slam algorithm is to get the R matrix which is the noise in the measurments.

and i cannot find to find any documentation regarding that matter. the only thing related to sensor noise i found is the resolution of the IMU Sensor BMI055 , but nothing on the other sensor, and of course the pixhawk integrates the 2 imu units, so this changes the data specified in the datasheet for the BMI055.

so does anyone have any experience in that matter?